Yeah, the full story is often a bit more complicated 
One way is use a layer mask. see: https://docs.gimp.org/2.10/en/gimp-layer-mask-add.html and https://www.gimp.org/tutorials/Layer_Masks/
Two layers of the same image. Bottom layer is edited to the black / white version Top layer is masked to provide 'windows' to show only the parts needed. Layer -> Mask -> Add Layer Mask (from selection)
This is quick and rough, I can just get it into 1 minute demo: https://imgur.com/k0wAXAs.mp4
Note the selection tools are in addition mode. Once you have the layer mask, adjust it with paint tools / more selections
black - hides / white - shows.
If you use a mask and save as a Gimp .xcf file, there is always the opportunity to return at a later date for further editing.
One way is use a layer mask. see: https://docs.gimp.org/2.10/en/gimp-layer-mask-add.html and https://www.gimp.org/tutorials/Layer_Masks/
Two layers of the same image. Bottom layer is edited to the black / white version Top layer is masked to provide 'windows' to show only the parts needed. Layer -> Mask -> Add Layer Mask (from selection)
This is quick and rough, I can just get it into 1 minute demo: https://imgur.com/k0wAXAs.mp4
Note the selection tools are in addition mode. Once you have the layer mask, adjust it with paint tools / more selections
black - hides / white - shows.
If you use a mask and save as a Gimp .xcf file, there is always the opportunity to return at a later date for further editing.